(1) Field of the Invention
The present invention relates to an image forming apparatus which reproduces a color image by superimposing multiple images of different colors reproduced based on a color image signal. More detailedly, the present invention relates to an image forming apparatus wherein a registration pattern for correcting color misregistration is formed on the actual printing medium so as to determine the conveyed state of the printing medium in the transfer step, mechanical problems (conveyance irregularity) in the transfer portion etc.
(2) Description of the Prior Art
Color image forming apparatuses have been known in which a color image is picked up as image data of Y(yellow), M(magenta), C(cyan) and Bk(black) and the image data of different colors are transferred to their corresponding recording portions to be reproduced as color separations and the color separated images are sequentially superimposed to thereby reproduce a color image.
Color image forming apparatuses of this type suffer from the problem in that a correct color image cannot be reproduced if each color separated image cannot be exactly laid over the others. Therefore, how this problem is solved is the key to the technical development of this field.
The main factors of misregistration of superimposition of different color separations can be described as follows:
That is, the main factors include: driving irregularities occurring during rotational drivings of the photosensitive members on which different color-separated images are recorded; inclination of the laser beam, as it sweeps to record an image on the photosensitive member, in the main scan direction (the axial direction of the photosensitive member) with respect to the photosensitive member; deviation of the point to be recorded on the photosensitive member by the laser beam with respect to the rotational direction of the photosensitive member (the auxiliary scan direction); and rotational driving irregularities of the printing media conveying belt for conveying printing media.
Conventionally, in order to assure superimposition of the images reproduced by coloring materials of different colors (Y, M, C and Bk), the above mechanical error causing factors (irregularities and fluctuations in various parts) were attempted to be eliminated by making adjustment to mechanical parts in assembly.
However, there is a limit to the mechanical countermeasures (adjustment). So, the final adjustment needs electric control such as timing control of start of writing the image by the laser beam.
As a usually used example, a means has been known which forms trial registration patterns of the colors on the image transfer-conveyance belt, reads the toner images of these registration patterns by means of an optical sensor so as to check the positional relationship between the toner images and compensates for the positional errors of the image formed, produced by the individual, color image forming portions (Japanese Patent Application Laid-Open Sho. 63 No. 286864).
However, in a configuration as above where registration patterns are formed on the transfer belt, there was a problem in that the positions of the registration patterns could not be determined correctly by the optical sensor.
This is because it is difficult to determine whether light incident to the optical sensor is the reflected light from the surface of the image transfer-conveyance belt or the reflected light from the registration patterns of the toner image (registration adjustment pattern image). Though the detected output signal from the sensor is processed through amplifiers, filters, and the like, it is still difficult to acquire correct information because of influence of noise, signal fluctuations, etc.
Another method has been devised and proposed in which the registration patterns of the component colors are actually transferred and formed on the printing medium onto which the final color image is to be reproduced so that correction information for reproducing the image of each component color is obtained based on the state of the forming position of the registration pattern for the component color (see Japanese Patent Application Laid-Open Hei 9 No.30051).
As stated above, color image forming apparatus involves a number of error (color misregistration) causing factors in various mechanical parts. So, assuming that errors of different parts exhibit the greatest large combined effect, the means disclosed in Japanese Patent Application Laid-Open Hei 9 No.30051, needs to have a number of timing adjustment patterns prepared beforehand and determine one ideal registration pattern from those patterns.
Therefore, if the system involves a large number of error causing factors and/or if some factors produce errors of distinct values, a large number of registration patterns need to be prepared beforehand for error estimation, which necessitate a lot of time and labor for data setting, printing etc., as well as a time consuming process for error determination. Moreover, it might happen that one sheet of paper is not enough to print the registration pattern, posing a problem of handling inconvenience.